The present invention relates to an inspection probe used for inspecting a board in a non-contact manner, and an inspection method and apparatus using this probe. A target board is represented by a board printed with conductive patterns at a small pitch and includes e.g., a flexible board (a "flexible board" includes an LSI package which is not mounted with IC chip and is to be mounted therewith, and will be referred to simply as a "circuit board" hereinafter). More particularly, the present invention relates to a non-contact board inspection probe and an inspection method and apparatus, all of which are suitable for inspecting local patterns on a board for disconnections and the like.
The board inspection probe and the inspection method and apparatus of the present invention are effective in inspecting a so-called bare circuit board on which no circuit elements such as IC packages are mounted yet although conductive patterns having a small pitch are printed thereon.
In conventional board inspection, if a board on which conductive patterns having a small pitch are printed has a large pitch on the electrode side, as shown in FIG. 1, probes can be brought into contact with the electrode groups (two or more electrode groups) of the board to energize the board (power is supplied from one electrode group, and the inspection result is detected on the other electrode group).
A recent highly-integrated circuit board, however, has small pitches in not only conductive patterns, but also electrodes. This makes it difficult to accurately bring probes into contact with the electrodes having a small pitch. An inspection for determining defectiveness/nondefectiveness (particularly, the presence/absence of a disconnection) of such a board having patterns (conductive paths) with a small pitch has often relied on visual observation or the like.
In recent years, the conductive patterns of a board (inspection target board) have a higher density (smaller pitch) along with decreases in size and weight of electronic devices. The decrease in pitch tends to cause disconnections in conductive patterns. A strong demand has therefore arisen for board inspection meeting this tendency. Demands for improving workability and reliability and decreasing the cost have become important.
In inspection for a board having patterns with a small pitch, in addition to a problem posed by the difficulty in accurate positioning of probes on electrodes, another problem is posed by an increase in the number of measuring points. More specifically, in such a board, when the wiring density of conductive patterns increases (i.e., when the pitch becomes small), the number of input and output points (the number of measuring points) increases. Even if contact probing is possible, it is technically difficult to maintain stable contact precision and contact properties. In addition, as the test conditions are becoming stricter than before, complicated, high-precision inspection jigs must be prepared, resulting in high cost.
Under these circumstances, several prior-art techniques based on non-contact probing, i.e., the board inspection free from the problem posed by contact between probes and electrodes are known.
For example, British Patent No. GB2143954A has proposed a technique for positioning a probe electrode at the end of a conductive path to form capacitive coupling between the electrode and the end of the conductive path. An AC signal is applied between the electrode and the one end of the conductive path, and a signal is detected at the other end of the conductive path through the above capacitive coupling. By this technique, a board can be inspected without bringing the probe into contact with the conductive pattern.
Japanese Patent Laid-Open No. 6-34714 (U.S. Pat. No. 5,254,953) is deemed an improved proposal of the non-contact inspection method disclosed in GB2143954A described above.
Japanese Patent Laid-Open No. 5-264672 (U.S. Pat. No. 5,274,336) discloses a capacitive coupling probe (probe chip) used in an in-circuit test for a high-density circuit board.
In the above prior arts, the "non-contact" means coupling free from ohmic contact and is equivalently used as the "capacitive". That is, a means for capacitive coupling is a capacitor.
The present inventor found that when the above prior-art inspection method and apparatus, however, were applied to a circuit board such as a bare board prior to mounting circuit parts thereon, it was difficult to highly accurately detect the presence/absence of a defect (e.g., a disconnection). That is, even if the prior-art technique is used to inspect a board in which the presence of a disconnection has been confirmed, an inspection result representing the absence of a disconnection is obtained. The present inventor found the cause for this as follows.
FIG. 2 is a block diagram of an inspection apparatus in U.S. Pat. No. 5,254,953. This prior-art technique is an apparatus serving as an in-circuit tester. This tester inspects to find whether a lead wire 111 of an IC package 110 is normally connected to a lead wire 140 on a circuit board by soldering 200. That is, the tester inspects soldered portions, but does not inspect the pattern itself for any defects.
Referring to FIG. 2, an AC signal is supplied from an oscillator 100 to the lead wire 140 between a probe electrode 120 and the lead wire 111 through a capacitor layer formed by air layer and the IC package 110. A shield 130 is arranged to prevent the probe electrode 120 from picking up EMI (Electro-Magnetic Interference) from various devices (not shown) located above the probe electrode 120.
If soldering 200 is proper, the AC signal is detected by an electrode 310 and measured by an inspection apparatus 300. Whether soldering is defective or nondefective is determined by the magnitude of the signal detected by the electrode 310. Note that the capacitance of the capacitor layer formed by the air layer and the IC package 110 between the probe electrode 120 and the lead wire 111 is as small as several femtofarad (fF), and the amplitude of the signal detected by the electrode 310 is very small.
The present inventor found that when this probe electrode 120 was applied to a bare board 500, as shown in FIG. 3, the measurement of a signal by the electrode 310 upon intentionally forming a disconnection 510 in a lead wire 520 on the board 500 had almost no difference in the amplitude of the detection signal from the measurement of a signal by the electrode 310 through a lead wire 520 free from disconnections.
According to the findings of the present inventor, no difference was found in detection signal between the cases in which the disconnection 510 was present and it was absent because the signal applied to the probe electrode 120 propagated in the electromagnetic field formed in the air layer and was received by a lead wire portion 520a, and the signal on the lead wire portion 520a was detected by the electrode 310.
Although the inspection apparatus in FIG. 3 has the shield 130 which is effective to protect the probe electrode 120 from the EMI signal coming from above, the inspection apparatus is defenseless against radiant waves from various radiant sources located below the electrode 120.
In inspecting a bare board, as shown in FIG. 3, the probe electrode must particularly come closer to the bare board. In the inspection apparatus disclosed in U.S. Pat. No. 5,254,953 to inspect an in-circuit board which need not bring a probe electrode closer to the board due to the presence of parts, the problem posed by the EMI signal from the board does not arise because the probe electrode is used far away from the board.